The present invention relates to a bright telephoto lens having a small telephoto ratio with a secondary spectrum properly compensated for which would largely affect the image forming ability of the telephoto lens.
It is known in the art of telephoto lenses that even if chromatic aberrations are compensated for with respect to light rays of two particular wavelengths, chromatic aberrations remain for light rays of other wavelengths, that is, the amount of the secondary spectrum influences the image forming ability to a large degree. Such secondary spectrum can be reduced in telephoto lenses having a convergent lens group and a divergent lens group arranged in order from the object by using a special optical material of anomalous dispersion such as an optical crystal such as fluorite, or glass of anomalous dispersion for positive lenses in the convergent lens group. The optical material of anomalous dispersion used is located most closely to the object since the more apart the light ray is spaced from the optical axis, the more effective against the chromatic aberration.
The optical material of anomalous dispersion, however, is soft, has a surface refractive index that varies widely with temperature, and is of a high coefficient of thermal expansion, so that the material is liable to be injured when placed most closely to the object in direct contact with ambient air, and to get damaged or have its image forming ability reduced due to temperature changes.